A torque transfer system is used conventionally to transfer torque of an internal combustion engine or a rotary electric machine to an axle (drive shaft) of a hybrid vehicle. In a torque transfer system disclosed in patent document (JP-B-3958220), for example, a torsional damper is provided between a crankshaft of an internal combustion engine and a power split mechanism. The torsional damper reduces transfer of torque variation of the crankshaft, which is transferred to the power split mechanism.
In the torque transfer system according to the patent document, the torque variation of the crankshaft often increases when rotation of the internal combustion engine is not stable, particularly when the internal combustion engine is started for example. When the torque variation increases, the torsional damper tends to clatter or clatter and the gears of the power split mechanism hit each other generating tooth hammer sound. The torque transfer system therefore controls the torque of the rotary electric machine, which is coupled to the torsional damper through the power split mechanism, in accordance with the torque variation of the crankshaft so that the torque variation and the tooth hammer sound is reduced.
The torque transfer system according to the patent document estimates the torque variation of the crankshaft based on a map data, which is determined in accordance with an angular position of the crankshaft and a coolant temperature of the engine. The torque transfer system controls the torque of the rotary electric machine based on the estimated torque variation thereby to reduce the torque variation of the crankshaft and the tooth hammer sound of the power split mechanism. The power split mechanism sometimes generates the tooth hammer sound, when an actual torque variation exceeds the estimated torque variation or increases excessively because of resonance in such an occasion as an irregular operation of the internal combustion engine. It is generally not possible to estimate or detect in real time the torque variation of the crankshaft. If a torque meter or the like, which can detect the torque variation in real time, is used, it will add cost.